Abstract
Research and/or Engineering Questions/Objective
The early stage of flame kernel formation and development have a significant influence on the combustion and emission characteristics in spark ignition engine. In this respect, conventional spark ignition system has disadvantages in terms of combustion instability and knocking. To mitigate these problems, new type of ignition system should be developed. The objective of this study was to develop a microwave-assisted plasma ignition system and investigate the effects of microwave-assisted plasma ignition on flame development in a constant volume combustion chamber (CVCC).
Methodology
The microwave-assisted plasma ignition system consisted of a signal control part, a signal transmission part and an ignitor. A magnetron from conventional microwave oven was utilized for microwave generation. The ignition was occurred by commercial high voltage ignition coil and then microwave was ejected to end of the spark plug. For the igniter, non-register spark plug was modified. These systems were combined and installed in a 1.4 L constant volume combustion chamber. Acetylene and air were mixed in a pre-mixing chamber, and were then supplied to the CVCC through intake valve. The initial equivalence ratio and in-chamber pressure were controlled by adjusting gas mixture before the ignition. The in-chamber pressure was acquired using a piezo-electric pressure transducer and a charge amplifier with resolution of 50 kHz. Based on the in-chamber pressure trace, heat release analysis were performed. To investigate the flame kernel development, high speed imaging was conducted using Shadowgraph.
Results
The lean limit was extended to equivalence ratio of 0.5 by applying microwave-assisted plasma ignition. The calculated flame rise time and flame development time were also advanced due to the faster flame kernel development with microwave ejection. However, the combustion enhancement by microwave ejection was seen to be diminished in rich mixture and high initial ambient pressure conditions. The combustion index was generally increased with new system.
Limitations of this study
In this study, the igniter was not optimized for the microwave ejection because the modified conventional spark plug was utilized. The efficiency of microwave transmission was about 30%. To maximize the effects of microwave ejection on combustion, optimized igniter should be considered.
What does the paper offer that is new in the field including in comparison to other work by the authors?
The researches about microwave-assisted plasma ignition for spark ignition engine are rarely done. This fields receive much attention in these days as the misfire and knocking make problems in engines. The content related to construct microwave-assisted plasma ignition system and igniter would provide new knowledge to readers.
Conclusions
New ignition system using microwave-assisted plasma was developed and investigated in a CVCC. The system showed enhanced ignition and combustion compared to conventional spark ignition system.
Key Words : Microwave, Non-thermal plasma, Flame kernel, Ignition, Combustion